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M. S. Pierce,
J. E. Davies,
J. J. Turner,
K. Chesnel, E. E. Fullerton,
J. Nam,
R. Hailstone,
S. D. Kevan,
J. B. Kortright,
Kai Liu,
L. B. Sorensen,
B. R. York,
O. Hellwig
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ABSTRACT: Using a combination of resonant soft x-ray scattering, magnetometry, x-ray
reflectivity and microscopy techniques we have investigated the magnetic
properties and microstructure of a series of perpendicular anisotropy Co/Pt
multilayer films with respect to structural disorder tuned by varying the
sputtering deposition pressure. The observed magnetic changes in domain size,
shape and correlation length originate from structural and chemical variations
in the samples, such as chemical segregation and grain formation as well as
roughness at the surface and interfaces, which are all impacted by the
deposition pressure. For low pressure samples we find evidence of a random
"gas-like" distribution of magnetic domains, while in the higher pressure
samples the domain structure exhibits only short range "liquid-like" positional
ordering. The structural and chemical disorder induced by the higher deposition
pressure first leads to an increase in the number of magnetic point defects
that limit free domain wall propagation. Then, as the sputtering pressure is
further increased, the domain wall energy density is lowered due to the
formation of local regions with reduced magnetic moment, and finally
magnetically void regions appear that confine the magnetic domains and
clusters, similar to segregated granular magnetic recording media.
01/2013;
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ABSTRACT: Interest in the magnetic properties of the ordered equiatomic alloy FeRh has been revived in recent years due to potential applications in heat assisted magnetic recording. This is based on the existence of a first order hysteretic phase transition from strongly anisotropic antiferromagnet (AF) to relatively isotropic ferromagnet (F) upon warming to ∼370 K. Here we investigate this transition, which shows significant coupling between structural, magnetic, and electronic degrees of freedom, via wide temperature range magnetotransport and magnetometry measurements on ordered epitaxial MgO(001)/FeRh(1000 Å) films. Consistent with bulk measurements, a large decrease in resistivity (∼30%) occurs on warming through the transition. The transition temperature shifts with applied magnetic field at −0.75 K/kOe due to the field-induced stabilization of the F phase, leading to large negative magnetoresistance (∼40% in 90 kOe) in the transition region (340–400 K). Isothermal field cycles reveal consistent behavior where the resistivity is controlled by the magnetization changes as the AF/F phase boundary is crossed. In the F state at high temperature anisotropic magnetoresistance (AMR) is observed (maximum amplitude ∼0.45%), in addition to high field negative MR due to field-induced suppression of electron-magnon scattering. The temperature dependence of the AMR reveals clear two-phase coexistence in the hysteresis region, providing a sensitive probe of remnant F regions upon cooling through the transition.
This behavior, and the accompanying coercivity enhancement, is discussed in terms of possible two-phase microstructures across the magnetic phase transformation.
Journal of Applied Physics 04/2011; 109(8):083913-083913-7. · 2.17 Impact Factor
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ABSTRACT: We have used resonant, coherent soft x-ray scattering to measure wave vector resolved magnetic domain memory in Co/Pd multilayers. The technique uses angular cross correlation functions and can be applied to any system with circular annuli of constant values of scattering wave vector q . In our Co/Pd film, the memory exhibits a maximum at q =0.0384 nm <sup>-1</sup> near initial reversal that decreases in magnitude as the magnetization is further reversed. The peak is attributed to bubble domains that nucleate reproducibly near initial reversal and which grow into a labyrinth domain structure that is not reproduced from one magnetization cycle to the next.
Applied Physics Letters 04/2011; · 3.84 Impact Factor
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ABSTRACT: For a fixed 2 μm×2 μm area of a Co/Pt-CoO perpendicular exchange bias system we image the ferromagnetic (FM) domains for various applied fields with 10-nm resolution by magnetic force microscopy (MFM). Using quantitative MFM we measure the local areal density of pinned uncompensated spins (pinUCS) in the antiferromagnetic (AFM) CoO layer and correlate the FM domain structure with the UCS density. Larger applied fields drive the receding domains to areas of proportionally higher pinUCS aligned antiparallel to FM moments. The data confirm that the evolution of the FM domains is determined by the pinUCS in the AFM layer, and also present examples of frustration in the system.
Physical Review Letters 11/2010; 105(19):197201. · 7.37 Impact Factor
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ABSTRACT: We report soft x-ray speckle metrology measurements of microscopic return point and complementary point memory in Co/Pd magnetic films having perpendicular anisotropy. We observe that the domains assemble into a common labyrinth phase with a period that varies by nearly a factor of two between initial reversal and fields near saturation. Unlike previous studies of similar systems, the ability of the film to reproduce its domain structure after magnetic cycling through saturation varies from loop to loop, from position to position on the sample, and with the part of the speckle pattern used in the metrology measurements. We report the distribution of memory as a function of field and discuss these results in terms of the reversal process.
New J. Phys. 01/2010; 12(3):035009.
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ABSTRACT: The behavior of minor hysteresis loops in perpendicular anisotropy [Co/Pt] and [Co/Pd] multilayers has been investigated. Upon applying a succession of identical magnetic field cycles, we observe a very substantial cumulative growth of the minor loop area. For the [Co/Pt] multilayers this effect only saturates near complete magnetization reversal while the behavior is slightly more limited for [Co/Pd] multilayers. We also find this cumulative growth to occur even if the minor loop field cycles are made asymmetric by means of a positive bias field. The cumulative behavior persists up to a sample-dependent threshold value above which this effect disappears. In all samples, the cumulative minor loop growth is correlated with a small reduction in the maximum magnetization value in each cycle. Magneto-optical Kerr microscopy studies correlate the minor loop growth with the memory and cumulative expansion of lateral domain cycling. All experimental observations can be consistently explained as an accumulation of small nucleation domains that aid subsequent reversals and facilitate the cumulative minor loop growth.
Physical Review B 01/2010; 82(10). · 3.69 Impact Factor
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ABSTRACT: In order to adjust the reversal field of high anisotropy [Co(2.8 Å)/Pd(9 Å)]8 bit patterned media (BPM), one may increase the individual Co thickness to change the overall anisotropy or alternatively combine the high anisotropy multilayer with a lower anisotropy material, thus creating a heterogeneous laminated system. In the current study, we find that [Co/Pd]8−N[Co/Ni]N hard-soft laminated BPM allows tuning the coercivity while maintaining a narrow normalized switching field distribution (σSFD/HC), whereas homogeneous structures with increased individual Co thickness show a more pronounced increase in σSFD/HC. Possible reasons for the different behavior, such as changes in strain and texture, are discussed.
Applied Physics Letters 12/2009; 95(23):232505-232505-3. · 3.84 Impact Factor
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ABSTRACT: We performed high field torque magnetometry measurements on CoO/[Co/Pt] magnetic multilayers that exhibit perpendicular exchange bias. We find that the antiferromagnet CoO layers strongly modify the uniaxial anisotropy of the multilayer structures. The strongest effects due to the CoO layers occur in the vicinity of the Néel temperature, where we observe a suppression of the first-order anisotropy and a smaller enhancement of the second-order anisotropy. This results in a nonmonotonic variation of the anisotropy with temperature and for selected samples a transition from perpendicular to in-plane and back to perpendicular anisotropy with increasing temperature.
Applied Physics Letters 10/2009; 95(13):132509-132509-3. · 3.84 Impact Factor
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ABSTRACT: We report microscopic mechanisms for an unusual magnetization reversal behavior in Co/Pt multilayers where some of the first-order reversal curves protrude outside of the major loop. Transmission x-ray microscopy reveals a fragmented stripe domain topography when the magnetic field is reversed prior to saturation, in contrast to an interconnected pattern when reversing from a saturated state. The different domain nucleation and propagation behaviors are due to unannihilated domains from the prior field sweep. These residual domains contribute to random dipole fields that impede the subsequent domain growth and prevent domains from growing as closely together as for the interconnected pattern. Comment: 13 pages, 3 figures, to appear in APL
Applied Physics Letters 07/2009; 95(2):022505. · 3.84 Impact Factor
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ABSTRACT: We report observations of nonlinear and hysteretic exchange bias in antiferromagnetically coupled hard/soft ferromagnetic bilayers. In such systems, the hard ferromagnetic layer acts as a tunable biasing layer replacing the antiferromagnet used in conventional exchange bias structures. It is shown that the reported behavior is due to the reciprocity of the exchange bias effect in conjunction with the presence of locally varying interfacial exchange which causes a preferential magnetization of the weakly exchange-coupled tuning regions during the exchange bias setting stage.
Phys. Rev. B. 12/2008; 78(22).
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ABSTRACT: An attractive approach for bit patterned media fabrication is the blanket deposition of magnetic material onto prepatterned substrates with elevated pillars and recessed trench areas. One issue with this method is the residual magnetic material in the trenches that causes disturbing stray fields during writing and readback. Here we present a technique to suppress the magnetic moment in the trenches with an additional annealing step by using prepatterned substrates consisting of SiN pillars on a Si wafer. The annealing triggers an interdiffusion process between the magnetic media and the Si in the trenches that results in the formation of a nonmagnetic silicide, while the magnetic moment on top of the SiN pillars remains substantially unaltered.
Applied Physics Letters 11/2008; 93(19):192501-192501-3. · 3.84 Impact Factor
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ABSTRACT: We show the possibility of creating magnetic domain memory in thin ferromagnetic films by inducing spatially varying exchange coupling interactions. We evidence this phenomenon in a perpendicular exchange bias film made of [Co/Pd] IrMn multilayers. Our coherent x-ray magnetic scattering speckle correlation study shows that the film exhibits no memory at room temperature but acquires a very high degree of magnetic memory, above 80% with subsequent field cycling when the sample is zero-field cooled below the blocking temperature of the IrMn layers (T<275 K).
Phys. Rev. B. 10/2008; 78(13).
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ABSTRACT: We have studied the recording performance of perpendicular exchange spring layer (ESL)-media for hard disk drive recording. In particular, we investigated the role of interlayer coupling by varying the thickness of a nonmagnetic coupling layer (CL). We demonstrate that not only the media writeability is improved upon optimizing the CL thickness, but also that substantial recording performance improvements can be achieved due to improved media noise properties. The potential of these media structures for high areal density recording is demonstrated by performing areal density measurements, which showed a substantial improvement for optimally coupled ESL-media.
Applied Physics Letters 09/2008; 93(12):122502-122502-3. · 3.84 Impact Factor
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ABSTRACT: The origin of the exchange bias (EB) effect has been traced back to the existence of pinned uncompensated spins (UCS) in the antiferromagnet (AFM) or at its interface. However, the understanding of the underlying mechanism is still clouded by contradictory reports: For example, both a parallel as well as an antiparallel orientation of the UCS relative to the magnetization direction of the ferromagnet (FM) were reported for similar FM/AFM systems. Here different magnetization histories in magnetometry and magnetic force microscopy measurements are used advantageously to demonstrate the co-existence of pinned UCS that are parallel and antiparallel to the cooling field in metallic (IrMn) and oxidic (CoO) EB systems. We further conclude that the EB effect is mainly a result of pinned interfacial UCS, which are antiparallel to the FM spins.
EPL (Europhysics Letters) 11/2007; 81(1):17001. · 2.17 Impact Factor
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ABSTRACT: A critical requirement for bit patterned media applications is the control and minimization of the switching field distribution (SFD). Here, we use the ΔH(M,ΔM) method to separate dipolar interactions due to neighbor islands from the intrinsic SFD by measuring a series of partial reversal curves of perpendicular anisotropy Co/Pd based multilayer films deposited onto prepatterned Si substrates. For a 100-nm-period island array the dipolar broadening contributes 22% to the observed SFD. For a 45-nm-period array this value increases to 31%. These results highlight the importance of quantifying long-range dipolar interactions for determining the intrinsic SFD of patterned media.
Applied Physics Letters 04/2007; 90(16):162516-162516-3. · 3.84 Impact Factor
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M. S. Pierce,
C. R. Buechler,
L. B. Sorensen,
S. D. Kevan,
E. A. Jagla,
J. M. Deutsch,
T. Mai,
O. Narayan,
J. E. Davies,
Kai Liu,
G. T. Zimanyi,
H. G. Katzgraber,
O. Hellwig, E. E. Fullerton,
P. Fischer,
J. B. Kortright
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ABSTRACT: Beautiful theories of magnetic hysteresis based on random microscopic disorder have been developed over the past ten years. Our goal was to directly compare these theories with precise experiments. To do so, we first developed and then applied coherent x-ray speckle metrology to a series of thin multilayer perpendicular magnetic materials. To directly observe the effects of disorder, we deliberately introduced increasing degrees of disorder into our films. We used coherent x rays, produced at the Advanced Light Source at Lawrence Berkeley National Laboratory, to generate highly speckled magnetic scattering patterns. The apparently “random” arrangement of the speckles is due to the exact configuration of the magnetic domains in the sample. In effect, each speckle pattern acts as a unique fingerprint for the magnetic domain configuration. Small changes in the domain structure change the speckles, and comparison of the different speckle patterns provides a quantitative determination of how much the domain structure has changed. Our experiments quickly answered one long-standing question: How is the magnetic domain configuration at one point on the major hysteresis loop related to the configurations at the same point on the loop during subsequent cycles? This is called microscopic return-point memory (RPM). We found that the RPM is partial and imperfect in the disordered samples, and completely absent when the disorder is below a threshold level. We also introduced and answered a second important question: How are the magnetic domains at one point on the major loop related to the domains at the complementary point, the inversion symmetric point on the loop, during the same and during subsequent cycles? This is called microscopic complementary-point memory (CPM). We found that the CPM is also partial and imperfect in the disordered samples and completely absent when the disorder is not present. In addition, we found that the RPM is always a little larger than the CPM. We also studied the correlations between the domains within a single ascending or descending loop. This is called microscopic half-loop memory and enabled us to measure the degree of change in the domain structure due to changes in the applied field. No existing theory was capable of reproducing our experimental results. So we developed theoretical models that do fit our experiments. Our experimental and theoretical results set benchmarks for future work.
Phys. Rev. B. 04/2007; 75(14).
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Journal of Applied Physics 01/2007; 101:09D102. · 2.17 Impact Factor
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ABSTRACT: We describe a method for the experimental characterization of intrinsic switching field distributions in perpendicular media. The method is based upon a comparison between the major loop and a set of recoil loops, which start at a certain distance ΔM away from saturation. In particular, we measure the applied field difference ΔH between the recoil loops and the major loop at identical M values, for which the average demagnetizing fields are the same. By analyzing complete ΔH(M,ΔM) data sets taken from multiple recoil loops, we gain a detailed measure of the intrinsic switching field distribution D(H<sub>S</sub>). Using polar-Kerr effect measurements of recording disk samples, we demonstrate the capabilities of our approach.
IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
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ABSTRACT: We have investigated the magnetic reversal and recording properties of layered ferromagnetically exchange coupled (or exchange spring) structures made of a hard layer and a soft layer. The hard layer is a longitudinal antiferromagnetically-coupled (AFC) media which is exchange coupled to a soft CoCrX layer via a CoRu interlayer. We find that optimizing the CoCrX and CoRu layer thicknesses is critical to achieve optimum improved writability while maintaining the media signal to noise ratio (SNR). Furthermore although we successfully decrease substantially the media coercive field (Hc) of these exchange spring structures, the energy barrier K<sub>u</sub>V appears unchanged. Finally, we show that these structures have the potential of improved SNR.
IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
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Kai Tang,
D. Margulies,
A. Polcyn,
N. Supper,
Hoa Do,
M. Mirzamaani,
M. Doerner,
Xiaoping Bian,
M. Mercado,
Li Tang,
H. Rosen, E.E. Fullerton,
Ching Tsang,
R. Nimmagadda,
Qi-Fan Xiao
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ABSTRACT: Lamination of multiple isolated magnetic layers has been shown to be an effective method to significantly increase signal-to-noise ratio in longitudinal media. These laminated media, however, are accompanied by low overwrite and wide magnetic pulse width, mainly as a result of poor writing of the bit transitions in the magnetic layer further away from the head and an offset in the transition position in the multiple magnetic layers resulting from head field spacing loss. We have demonstrated that the transition writing and transition alignment in the multiple magnetic layers of the laminated antiferromagnetically coupled (AFC) media can be optimized by adjusting the magnetic anisotropy of the relevant magnetic layers to compensate for the reduction of the head field magnitude with spacing. Such optimization results in significant improvements in media recording performance, leading to successful application of this medium technology. In this paper, we will highlight some of these improvements and discuss our approaches to further improve the recording performance by reducing the thicknesses of the magnetic layers and the lamination spacer layer in the laminated AFC film stack and by introducing additional elements in the magnetic layer.
IEEE Transactions on Magnetics 03/2005; · 1.36 Impact Factor
